For SSTables 3.x. ('mc' format), range tombstones are represented by
their bounds that are written to the data file as so-called RT markers.
For adjacent range tombstones, an RT marker can be of a 'boundary' type
which means it closes the previous range tombstone and opens the new
one.
Internally, sstable_writer_m relies on range_tombstone_stream to both
de-overlap incoming range tombstones and order them so that when they
are drained they can be easily thought of as just pairs of their bounds.
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